We plan to isolate, purify and partially characterize the macromolecular bone resorption stimulating agent(s) known as osteoclast activating factor (OAF). The factor(s) will be isolated by physical and chemical methods from culture medium of phytohemaglutinin-stimulated human peripheral leukocytes and assayed for the ability to stimulate bone resorption in organ culture. Chemical characterization of the purified factor(s) will include initially amino acid and carbohydrate composition, end group analysis and, if sufficient material, amino acid sequence.